Optical systems have been developed for imaging fingerprints directly from the contact of a person's finger with a platen or prism. Such systems provide a means of creating fingerprint images for purposes of storage, analysis, or printing. These systems also avoid the time consuming and messy process of obtaining fingerprints by using ink and rolling the inked fingers across a printing surface.
Typically, as shown in FIG. 1 which depicts the geometry of the platen/finger interface for a typical fingerprint imaging apparatus, a finger to be imaged 10 is placed on the upper surface of an optically transparent prism, referred to as a platen 12. The surface topography of finger 10 can be approximated by a series of ridges, R with intermediate valleys, V. The ridges of finger 10 contact platen 12 while the valleys do not and instead serve to form the boundaries of regions of air and/or moisture.
The finger to be imaged is illuminated by a light source (not shown) located below platen 12. Light from the light source is incident on the finger-receiving surface of the platen at an angle of incidence, .theta..sub.I, measured with respect to a normal N to the platen surface. Light reflected from platen 12 is detected by an imaging system (not shown) which usually includes some form of a detector. The aperture of the imaging system determines which of the reflected light rays are detected. In this sense the orientation of the imaging system, i.e., the angle between the optical axis of the imaging system and the normal to the platen surface (referred to as the "observation angle") determines which rays of light reach the detector.
The components of a typical fingerprint imaging system are oriented so that the angle of observation is greater than the critical angle .theta..sub.AP for the interface between platen 12 and the air above the finger-receiving surface of platen 12, where the critical angle .theta..sub.AP is defined as the smallest angle of incidence for which light striking the platen/air interface is totally internally reflected within platen 12. The illumination source is then oriented so that the light rays it produces cover a range of incidence angles which includes the angle of observation of the imaging system. Since the angle of incidence is equal to the angle of reflectance, .theta..sub.R, the range of angles of reflectance also includes the angle of observation of the imaging system.
The value of the critical angle at the interface between two materials depends on the index of refraction of those materials. In this case, it depends on the indices of refraction of the platen material and air, and is defined according to Snell's law as follows: EQU .theta..sub.AP =Arcsine (N.sub.A /N.sub.P), where (1)
.theta..sub.AP is the critical angle, N.sub.A is the index of refraction for air, and N.sub.P is the index of refraction for the optical platen. The index of refraction for air, N.sub.A, is approximately 1 and the index of refraction of the platen material, N.sub.P, is typically 1.491 for acrylic plastic. With these values, the critical angle .theta..sub.AP for the platen/air interface has a value of approximately 42 degrees.
As noted, the critical angle .theta..sub.AP for the platen/air interface, based on the actual index of refraction of the platen material, forms a lower bound on the angle of observation. This in turn provides a limit on the angles of incidence and reflection, and hence the orientation of the illumination source. A further constraint arises because there is an incentive to observe the image at the smallest practical angle of observation (corresponding to the smallest practical angle of reflection, .theta..sub.R), as this reduces distortion due to image tilting. Thus, the angle of observation is typically chosen to be close to, but greater than the critical angle.
As indicated, by appropriate selection of the angles of incidence and observation, it can be arranged so that at those locations where air contacts the platen surface, i.e., in the valley regions V of finger 10, light is totally internally reflected. By way of example, light ray 14 is incident on platen 12 at an angle of incidence of .theta..sub.I with respect to the normal N to platen 12, where .theta..sub.I is in excess of the critical angle .theta..sub.AP for a platen/air interface. Therefore, light is totally internally reflected at the platen/air interface with the angle of incidence .theta..sub.I equaling the angle of reflection .theta..sub.R. In this case, light is not reflected off of the valley regions V of finger 10 because it is not refracted through the platen/air interface and thus is not incident on finger 10. The resulting image of the valley regions of finger 10 is bright since the incident light is reflected at the platen/air interface.
In those locations where the ridges R of finger 10 contact platen 12, total internal reflection does not occur. Instead, what is termed "frustrated total internal reflection" is found to occur. This is because the index of refraction of finger 10 is larger than that of air, so that the angle of incidence no longer corresponds to the critical angle for the relevant interface. As shown by light ray 16, which is incident on the surface of platen 12 at a location where a ridge R of finger 10 is contacting platen 12, the light is now refracted through the platen/finger interface where it is partially absorbed and partially dispersed upon contact with finger 10. In this case only a small fraction of the incident light is reflected back to a detector at an angle of reflection .theta..sub.R equal to the angle of incidence .theta..sub.I, and therefore the ridges R of finger 10 contribute a dark component to the image of the fingerprint.
When the reflected light is observed at an angle greater than the critical angle .theta..sub.AP (i.e., the angle of observation is greater than the critical angle), an image of the fingerprint can be formed on a detector. As mentioned, the image includes bright regions which correspond to the valleys V of finger 10 and darker regions which correspond to the ridges R of finger 10. After detection, the fingerprint image can be permanently recorded for storage and/or further analysis.
Alternatively, it is possible to produce a fingerprint image which is of the opposite polarity of that described above. Such an image would be one in which the ridges R of finger 10 contributed a bright region, while the valleys V contributed a dark region. To obtain such an image the light source used to illuminate finger 10 is placed along the normal N to platen 12, i.e., at an angle of incidence of approximately 0 degrees. Since this angle of incidence is less than the critical angle for a platen/air interface, a substantial amount of the incident light is refracted through the platen/air interface and is reflected in a diffuse manner off of finger 10. If the reflected light is observed at an angle greater than the critical angle for the platen/air interface, very little reflected light is detected. However, a small amount of the light which strikes the ridges R of finger 10 is reflected along the angle of observation due to dispersion. Accordingly, a fingerprint image is formed in which the ridges R of finger 10 are comparatively brighter than the valleys V.
It has been found that fingerprint image artifacts sometimes occur which lead to a degradation of the image. These artifacts take the form of some type of bridging in the image between adjacent ridges R of a person's finger. The problem is especially pronounced for individuals that have particularly moist hands, and thus appears to be related to the presence of moisture on the fingers of the person whose fingerprint is being taken. The bridging artifact can be reduced by drying the hands or by cleaning the hands with a solvent. However, it has been found that such drying and cleaning have a tendency to interfere with the formation of proper contact between the finger ridges and the platen surface, thereby preventing the formation of a clear image of the fingerprint.
It has been suggested to use a blow dryer to blow heated air onto the fingers of the person whose fingerprint is being imaged in order to reduce the degradation of the image caused by the presence of moisture. It has also been suggested to use a heated platen for the same purpose. However, both of these proposed solutions to the problem require the use of additional elements in the fingerprinting apparatus, and neither appears to work completely satisfactorily for persons who sweat profusely.
What is desired is an apparatus for obtaining fingerprints which is less sensitive to the presence of moisture on a person's finger and is capable of producing high quality fingerprint images in the presence of moisture.